Amplitude limiter circuits



Aug. 15, 1950 D. G. c. LUCK 2,519,057 AMPLITUDE LIMITER CIRCUITS FiledJuly 25, 1946 IN V EN TOR.

DavjdaGLu/ck AT TOR/VEY This invention relates Patented Aug. 15, 1950ALIPLITUDE LIMITEB CIRCUITS David G. C. Luck, Princeton, N. J., assignorto Radio Corporation of America, a corporation of Delaware ApplicationJuly 25, 1946, Serial No. 686,182

1 l to improvements in amplitude limiter circuits. Limitersfindapplication in a variety of systems, including radio receivers,television apparatus, radar, and phase and frefquency measuringequipment The function of ja limiter is to accept an alternating voltagewave iwhose amplitude may vary, so long as it is never less than apredetermined value) andcutofi the peaks of the wave at somepredetermined level, thus. providing an output wave which is' flattoppedand of substantially constant amplitude. The principal object of thepresent invention is to. provide methods of'and means for obtaininglimiter action more .nearly ideal than that exhibited by limitercircuits used heretofore.

Another object of the invention is to provide a voltage limiter circuitwherein the amplitude of the output wave is substantially unaffected byvariations in vacuum tube characteristics.

A. further object of the invention is to provide meansfor minimizing orpreventing phase shift in*the limiter output with variation in amplitudeof: the input wave.

The invention will be. described with reference to the accompanyingdrawing, wherein:

Figure l is a schematic. circuit diagram of a voltage limiter embodyingthe instant invention,

Figurez is an oscillogram Of a typical-input wave and the levels atwhich it is to be clipped;

' Figure 3 is an oscillogram of the anode current produced in one of thetubes of Figure 1 by the wave 01' Figure 2, r V Figure 4 is anoscillogram of the voltage at the anode of'said tube,

Figure 5 is an oscillogram showing the effect oi variation in inputlevel upon the clipping levels ina prior art limiter,

. :Figure 6 shows the anode current produced by the wave of Figure 5,and

t Figure 7 is an oscillogram of the output wave of aprior art limiterunder the conditions illustrated in Figures 5 and 6.

Referring to Figure 1, the input wave to be limited is applied through acoupling capacitor l and a resistor 3 to the control grid of an electrondischarge tube 5. The tube 5 is illustrated as a tube 5, and cathodebias is provided by means of resistors l3 and I5.

'l'he circuit thuis far described is substantially 4 Claims. (01.178-44) I that of a typical prior art limiter, and it operates asfollows: The input wave must be of at least suflicient amplitudetoexceed the bias (provided by resistors I3 and I5) 50 that the gridbecomes positive with respect to the cathode and draws current duringeach positive half cycle of the input wave, and so that the grid goesfar enough negative with respect to the cathode during negative halfcycles to cut oil the flow of plate current in the tube 5'. Figure 2illustrates these conditions. Ordinarily, the input amplitude is muchgreater than shown in Figure 2, i. e. of the order of ten times the gridbias. The bias should be adjusted so that grid current flow and platecurrent cut-ofi occur at equal distances above and below the A.-C.axisof the input wave, as shown in Figure 2.

The plate current of the tube 5 flowsinpulses, as shown in Figure 3,going to zero during negative half cycles of the input. During each positive half cycle, the plate current increases with increase in theinstantaneous voltage of the input wave, until the grid draws current.The grid current flows through the resistor 3. producingtherein a.voltage drop in opposition to the input voltage and thus tending toprevent the grid from going further positive.

The maximum positive potential at the grid is approximately equal to thepotential at the oath ode, and the plate current is substantiallylimited to a maximum value corresponding to zero gridto-cathode voltage,regardless of the amplitude of the input wave. By proper choice of theplate load resistor 7 and the screen grid bias,; plate saturation can bemade to occur at this point, so that the small residual increase of gridpotential has little effect on the maximum plate current. Thus the platecurrent of the tube5 is of substan-' tially rectangular wave form, asshown in Fig? ure 3. y The instantaneous voltage e at the anode of thetube 5 is the difference between the supply accordance with conventionalpractice, the amplitude of the output wave will remain fairly constantover a considerable range of variation of input amplitude; However,variations in the characteristic of the limiter tube 5 will change theoutput amplitude. Such variations result from-tube aging, fluctuationsin cathode temperature, etc. I

Ordinary tube variations will not affect the maximum valueor 6p, as longas the input amplis;

put terminal, but is connected instead to the grid of a tube H. The tubeI1 is provided wi tha grid leak resistor 9 returned to ground ands loadresistor 2| connected from the to-ground. The plate of the tube i1is-connected directly "to the B supply.

A further tube 23 is provided with its cathode connected to that of thetube f1 and its plate connected directly to 3+, thus placing the spacedischarge paths. of the tubes l1 and parallel. The :common :nathode loadresistor is connected to the limiter outputiermilie-.15.. Theeontrolgrid of the tube .23 mainaimed fi no predetermined positive.voltage .Ec with respect to ground potential .iby means .of a yoltagedivider comprising resistors 21] and 2:9 ponnected in series across the.fB? supply. .A filter capacitor .125 is provided across the ire $151101A9 to. preventEtzirom being varied by variation in the voltageat .thecathode of .ztheltube 223 as aresultof coupling through :the grid tocathode capaci ance of .the tube 23.

. The operation of the LtllbBS I] and v2,3. and .their asseciatedcircuit is as follows:

1 During ne ativehalfcyclesxof the inputs-ignal, the tube 5 is c t on,as .described above. -Ihe voltage e at the .grid of the tube l l is-then5.91116 ,definitefractionoi the plate supply .voltageEadepending onlyupon :the .ratioof resistors tend J9. Resistors .21 and 29 areproportioned so that the voltage Eeatthe grid of the .tube '23 islessthan e by an amount .equalsto the amplitude which the limiter outputvoltage is to have. .Aslong ase is :more .thaIl'Ec, :theatu-be Ll]opcrates as a cathode vfollower, conductin just enoughcurrent so thatthedrop in iill'leYlOad re sistor 2| is substantially equal to-e k-This'keeps thewothode of .thetube 23 positive with :respect to its :controlgrid, and the tube 123 is cutoi f. Since the cathdde .follower .:actionof "the tube substantially independentof ordinary variain tubecharacteristics, the voltage at the output terminals during.negativeihalf cycles of theinput .will be determined entirely.:by:thesupplyvoltageEtandithe resistors and i3.

'When the input signal is positive .at the grid of the :tube 5, .thedischarge :ipathi-of this tube acts 'as.a relatively low resistance inparallel theresistor 19. reduces theyvoltage e at the grid of the tube Hto a value considerably less than the bias .Eeon :the :tube :23. Thetube zpsnow acts as a :cathode followenrmaintaining a voltage .dropequalto Eeiacross themesistor 21, keeping the tube l1 cutoff. Thussubjectonly to the condition that th input amplitude .be sufficient .to cut offlathe .tube .i-rduring negative half cycles and :to cut off the tube 1]during positive half cycles, "the output amplitude .will be determinedsolelybythe plate supply voltage, the ratio of resistorsfll :and li),andthe'ratio.ofresistors 2.1 and 29. A difference must exist :betweenthe potential at the controlgrid and that at the cathode of a cathodefollower, i'n'iorder to provide the control of plate cur-rent necessaryfor cathode follower action. Thisdif- 4. ference in potential will varyslightly with variations in the characteristics of the tube. The neteffect of such variations in the described circuit is ne ligible, since,for the A-C. component of the signal, those of the tube l! aresubstantially cancelled by those of the tube 23.

In certain applications of limiter circuits, as inphase comparisonsystems like that described in copending patent application Serial No.647,599, now Patent No. 2,513,477, issued July 4, 1950, filed February14, 1946, by Samuel Gubin and entitled.Qmnidirectional Radio Ranges, it

necessary to prevent the axis of the input signal from being shiftedwith variations in input amplitude. According to the present invention,the control grid of the tube 5 is returned to ground through .a diode 3|and a resistor 33. In a prior art circuit, the diode 3| would be omittedand the grid returned directly through the resistor 33.

.The grid .of .the tube draws :curren-t :only' :during. positive halfcycles of the :input signal. Without the diode 31, ;the capacitor 1|will the: come charged, since the current flowing :tlmough it fromrightto 'leftiduring positive half cycles) willexceed that:flo.wing"trom cleft .to-right, by the grid current. This will occur tosome :extent even if .the resistors "is :made low with respect to theinput resistanceof-the tube 5.

,The capacitor I :will act like .an additional source of bias, in serieswith :the input signal, and have the effect of shifting the :axis of theinput in a negative direction. "This increases'sthe positive swingnecessary to .cause .grid current flow, and decreases-the negative swingrequired for out off,:as-shown iniliigure'fi.

If the voltage developed across the -.capacitor 1 were constant, ,itcould be compensated :by :an adjustment of the cathode zbias resistors13 and I5. However, some 'variationof. grid current does occur withvariation in inputamplitude, :and this causes corresponding variation,of the charge on the capacitor. Thus the condition shown in :Figure5,.or its converse, i. 'e. anupwardshiit of the axis, will occuras themplitude varies.

It is evident from Figure "5 that as the axis moves downward, cut offoccurs :earlier and ends later in the cycle than before. Grid currentstarts later and ends earier. The plate current wave, shown in Figure 6,f-is narrowed a't the"top and widened at the bottom. Iheoutput voltageWave (Figure 7) has its leading .edges advanced in phase and itstrailing edgesdelayed.

The diode 3| prevents the above-described shifts in the axis of theinput signal, with their resultant variations in phase of the output'signa-l, by equalizing the currents flowing through the capacitor Theresistor .33 is.sele.cted,':taking into consideration the conductivityof :thezdiode 3 1 ha the current .ilowin th ou h the diode du in a e :hit cyc es is equal to e r d current drawn by the tube 5 during pos U 935: the 1 1 2 1? ampl tud PQWQfifiQF 52 creases, both currents willincrease ordecrease accordingly, maintaining the average through thecapacitor l Tat zer and so prevent; ing'the accumulation of chargetherein Iclaim'as my'invention; l i A voltage am itude limiter om is ngelectron i eent cement theist e the d a al d "er-i n a de inclu i g a ineente in se g ith the e tral grid for applying between said control gridand i c t e a decal ha n alte nate sitiv and negative loops, meansbiassing said' control grid with respect to said cathode to a potentialsubstantially midway between zero and that required to cut ofi the flowof anode current in said tube, a load impedance for said tube, a sourceof anode voltage supply, a circuit connecting said source in series withsaid discharge tube and said load impedance, a second electron dischargetube including an anode connected to the positive terminal of saidsource, a control grid connected between said load impedance and saidfirst-mentioned tube to receive therefrom output signals with fiat loopsand flat troughs when the peak magnitude of said loops exceeds themagnitude of said biassing potential, and a cathode; a load resistorconnected between said cathode and the negative terminal of said source,means for deriving a final output signal from across at least a portionof said load resistor, a third electron discharge tube including ananode connected to the anode of said second tube, a cathode connected tothat of said second tube, and a control grid; and means for biassingsaid control grid to a potential substantially less than that of thepositive terminal of said supply source and substantially greater thanthe potential appearing at the anode of said first-mentioned tube whenthe potential of its control grid is near to that of its cathode.

2. The invention as set forth in claim 1, wherein said means forapplying said signal to said firstmentioned tube further includes acapacitor connected between said control grid and the source of saidsignal, including a unilateral conductor connected between said controlgrid and the cathode of said first tube in such polarity as to conductfrom said cathode to said grid.

3. In a voltage amplitude limiter including a first electron dischargetube connected to draw grid current and to cut off respectively inresponse to alternate positive and negative loops of an A.-C, input waveto thereby produce an output voltage wave having flat tops and flattroughs,

a system for preventing variation in the levels of said troughscomprising a second electron discharge tube connected as a cathodefollower, with a load resistor common to its input and output circuits,means for applying the output of said first tube to the input of saidsecond tube, a further tube with an anode and a cathode connected to theanode and cathode respectively of said second tube and a control grid,and means for biassing said control grid to a potential substantiallyhigher than that of said troughs in the output of said first. tube,whereby the minimum voltage across said load resistor is substantiallyindependent of variations in the characteristics of any of said tubes.

4. In a voltage limiter system, a pair of electron discharge tubes eachincluding at least an anode,

component between one of said control grids and said negative terminal;means for deriving an output signal from across at least a portion ofsaid common load resistor; and means for biassing said other controlgrid to a potential substantially positive with respect to the minimumpeak value of said first-mentioned signal and substantially y, negativewith respect to the average value of said signal.

DAVID G. C. LUCK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,276,565 Crosby Mar. 1'7, 19422,340,364 Bedford Feb. 1, 1944

